Bumper beam having ribs on several walls of the bumper beam

ABSTRACT

A bumper beam is made of at least one rolled formed steel sheet containing an upper beam and a lower beam extending in a transversal direction, the upper beam and the lower beam each having a closed cross-section defined by a front wall, a rear wall, an upper wall and a lower wall, the upper wall and the lower wall joining the front wall to the rear wall, each of the front walls of the upper beam and of the lower beam containing a front rib extending transversally and towards the interior of the bumper beam. At least one of the upper wall of the upper beam, the lower wall of the lower beam, the rear wall of the upper beam and the rear wall of the lower beam further containing a rib extending transversally and towards the interior of the bumper beam.

FIELD OF THE INVENTION

The present invention provides a bumper beam for an automotive vehicleof the type made of at least one rolled formed steel sheet comprising anupper beam and a lower beam extending in a transversal direction, saidupper beam and said lower beam each having a closed cross-sectiondefined by a front wall, a rear wall, an upper wall and a lower wall,the upper wall and the lower wall joining the front wall to the rearwall, wherein each of the front walls of the upper beam and of the lowerbeam comprises a front rib extending transversally and towards theinterior of the bumper beam.

The invention also provides to a method for producing such a bumperbeam.

BACKGROUND OF THE INVENTION

Bumper beams comprising an upper beam and a lower beam are known as“8-shaped” bumper beams when a central wall forms both the lower wall ofthe upper beam and the upper wall of the lower beam and as “B-shaped”bumper beams when a space extends between the lower wall of the upperbeam and the upper wall of the lower beam. Such bumper beams are knownfor having good strength and impact characteristics, thanks to the lowerwall of the upper beam and to the upper wall of the lower beam whichincreases the resistance of the bumper beam, while remaining relativelylight and being adapted to the available space in the vehicle where thebumper beam is to be installed. Documents U.S. Pat. No. 8,716,624 and US2014/0361558, for example, disclose 8-shaped bumper beams and documentWO-2016/046582, for example, discloses a B-shaped bumper beam.

A bumper beam is required to have a particular behavior in case of animpact, for example a pole test impact, wherein a localized obstaclehits the central part of the bumper beam at a speed around 15 Km/h. Moreparticularly, the bumper beam has to be able to be deformed whileabsorbing energy when the impact occurs such that the energy of theimpact is not or less transmitted to the parts extending behind thebumper beam in the vehicle.

To this end, the bumper beam has to have satisfactory characteristics interms of resistance to a peak effort greater than a predetermined effortthreshold, which is applied to the bumper beam during the impact, ofminimal energy absorbed after deformation of the bumper beam at apredetermined distance of deformation due to the impact and ofresistance to breaking during the deformation of the bumper beam whenthe peak effort is applied to the bumper beam as well as after animportant amount of deformation, meaning that the bumper beam has to beplastically deformed and not break over a predetermined distance ofdeformation to ensure absorption of the energy during the plasticdeformation.

When designing a new bumper beam, one tries to have the best results inthese three parameters (resistance to a peak effort, minimal energyabsorbed and resistance to breaking). However, trying to improve one ofthese characteristics is generally detrimental to one and/or the othercharacteristics. For example, increasing the resistance of the bumperbeam, for example by modifying the geometry of the bumper beam or byincreasing its tensile strength, such that it can resist to a greaterpeak effort makes the bumper beam less deformable and more likely tobreak before the deformation of the bumper beam reaches thepredetermined distance.

Furthermore, it is known to bend the bumper beam in the transversaldirection in order to obtain a curved bumper beam having an improvedbehavior and more adapted to the geometry of the vehicle. However,bending the bumper beam can cause the rear walls, formed by planarsurfaces of large dimensions, of the bumper beam to buckle. Thisbuckling creates waves in the planar surfaces which therefore do notremain planar after bending. This phenomenon is greater as the radius ofcurvature of the bumper beam is smaller. This buckling is problematicbecause the depth and height of the waves can be around or exceed theacceptable manufacturing tolerances. Consequently, the integration ofthe bumper beam with the surrounding components, for example withcrash-boxes arranged at the end of the bumper beam can be problematic.

BRIEF SUMMARY OF THE INVENTION

An object of the present invention is to provide a bumper beam havingimproved features in terms of behavior and/or in terms of quality of thebumper beam.

To this end, the present invention provides a bumper beam of theabove-mentioned type, wherein at least one of the upper wall of theupper beam, the lower wall of the lower beam, the rear wall of the upperbeam and the rear wall of the lower beam further comprises a ribextending transversally and towards the interior of the bumper beam.

Providing a rib in one of the rear wall allows for reducing thedimensions of the planar surfaces forming the rear wall on either sideof the rib by reducing the height of the planar surfaces. Indeed, theeffort to induce buckling of a surface becomes lower as the length ofthe surface in the elevation direction increases. As compared to thissituation wherein the rear wall extends in a single plane, sinceproviding a rib in the rear wall makes it possible to reduce the lengthof the planar surfaces in the elevation direction, buckling of theplanar surfaces can be avoided because the effort to induce buckling isgreater and remains inferior to the effort applied on the bumper beamduring the bending of the bumper beam. Consequently, thanks to the rearrib, buckling of the rear wall can be avoided during the bending of thebumper beam.

Providing a rib in one of the upper wall of the upper beam or in thelower wall of the lower beam, in addition to avoiding buckling, improvesthe performances of the bumper beam in terms of resistance to breakingand in terms of resistance to a peak effort by creating a furtherdeformation pattern during the impact.

In certain embodiments, the present invention is directed to a bumperbeam for an automotive vehicle made of at least one rolled formed steelsheet comprising an upper beam and a lower beam extending in atransversal direction, said upper beam and said lower beam each having aclosed cross-section defined by a front wall, a rear wall, an upper walland a lower wall, the upper wall and the lower wall joining the frontwall to the rear wall, wherein each of the front walls of the upper beamand of the lower beam comprises a front rib extending transversally andtowards the interior of the bumper beam, wherein at least one of theupper wall of the upper beam, the lower wall of the lower beam, the rearwall of the upper beam and the rear wall of the lower beam furthercomprises a rib extending transversally and towards the interior of thebumper beam.

In some embodiments, at least one of the rear walls of the upper beamand of the lower beam comprises a rear rib extending transversally andtowards the interior of the bumper beam.

In some embodiments, the rear rib has a curved cross-section having aradius of curvature greater or equal to 0.5 times the thickness of thesteel sheet.

In some embodiments, the rear wall on which the rear rib extendscomprises at least two planar surfaces extending on either side of therear rib, the position and/or the height of the rear rib being arrangedsuch that each planar surface has a height inferior or equal to half ofthe height of the rear wall on which the rear rib extends.

In some embodiments, the depth of the rear rib is superior or equal to0.5 times the thickness of the steel sheet and inferior than or equal tothe distance between the front wall and the rear wall on which said rearrib extends.

In some embodiments, the rear rib extends opposite a front rib.

In some embodiments, each of the rear walls of the upper beam and of thelower beam comprises a rear rib extending transversally and towards theinterior of the bumper beam.

In some embodiments, the upper wall of the upper beam and/or the lowerwall of the lower beam comprise a reinforcement rib extendingtransversally and towards the interior of the bumper beam.

In some embodiments, the reinforcement rib has a curved cross-sectionhaving a radius of curvature greater than or equal to 0.5 times thethickness of the steel sheet.

In some embodiments, the depth of the reinforcement rib is superior thanor equal to 0.5 times the thickness of the steel sheet and inferior orequal to a third of the distance between the upper wall of the upperbeam and the lower wall of the lower beam.

In some embodiments, the bumper beam is curved in the transversaldirection, the curvature radius of the bumper beam being less than orequal to 4000 mm.

In some embodiments, the steel sheet is made of a steel having a tensilestrength greater or equal to 980 Mpa.

In some embodiments, the steel sheet has a thickness substantiallycomprised between 0.8 mm and 1.5 mm.

In some embodiments, a central wall extending between the front wallsand the rear walls of the upper beam and of the lower beam forms boththe lower wall of the upper beam and the upper wall of the lower beam.

In some embodiments, the central wall extends in at least two differentplanes.

In some embodiments, a space extends between the lower wall of the upperbeam and the upper wall of the lower beam.

In some embodiments, the lower wall of the upper beam and/or the upperwall of the lower beam extends in at least two different planes.

In some embodiments, the bumper beam further comprises a reinforcementelement made of a steel sheet, said reinforcement element being attachedto the upper beam and to the lower beam such that the reinforcementelement extends opposite at least a part of the front walls of the upperbeam and of the lower beam and defines with said front walls at leastone cavity extending between said front walls and said reinforcementelement.

The invention also relates to a method for producing a bumper beam asdescribed above, comprising the steps of:

providing a steel sheet,

roll forming the steel sheet in successive rolling stations in order toform a bumper beam comprising an upper beam and a lower beam extendingin a transversal direction, said upper beam and said lower beam eachhaving a closed cross-section defined by a front wall, a rear wall, anupper wall and a lower wall, the upper wall and the lower wall joiningthe front wall to the rear wall, wherein each of the front walls of theupper beam and of the lower beam comprises a front rib extendingtransversally and towards the interior of the bumper beam, wherein atleast one of the upper wall of the upper beam, the lower wall of thelower beam, the rear wall of the upper beam and the rear wall of thelower beam is shaped in at least one of the rolling stations to comprisea rib extending transversally and towards the interior of the bumperbeam.

In some embodiments, the steel sheet is made of a steel having a tensilestrength greater or equal to 980 MPa.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects and advantages of the present invention will will beunderstood upon reading the following description, given by way ofexample and made in reference with the appended drawings, in which:

FIG. 1 is a perspective view of a bumper beam assembly comprising abumper beam according to the invention,

FIG. 2 is a cross-section view along plane II-II of FIG. 1,

FIG. 3 is a cross-section view along plane of FIG. 1,

FIG. 4 is a cross-section view of a bumper beam according to anotherembodiment of the invention,

FIG. 5 is a cross-section view of a bumper beam according to anotherembodiment of the invention,

FIG. 6 is a series of cross-sections showing a shape of a steel sheet ateach forming step when forming the bumper beam of FIG. 1,

FIG. 7 is an enlarged view of steps 4 to 7 shown in FIG. 5, and

FIG. 8 is a diagram showing the force applied to the bumper beamrelative to a distance of deformation of the bumper beam of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

In the description, the term “longitudinal” is defined according to therear-front direction of a vehicle and the term “transversal” is definedaccording to the left-right direction of the vehicle. The terms “up”,“upper”, “lower” are defined relative to the elevation direction of avehicle.

In reference to FIG. 1, a bumper beam assembly for an automotive vehicleis described. Such a bumper beam assembly is to be arranged at the frontand/or at the rear of a vehicle to protect the motor compartment and thevehicle compartment in case of a front and/or rear impact against thevehicle.

The bumper beam assembly comprises a bumper beam 1 extendingsubstantially transversally and two crash-boxes 2 extending in thelongitudinal direction attached to the bumper beam 1 in the vicinity ofeach transversal ends 4 of the bumper beam 1. An attachment plate 6 isprovided at the end of each crash-box 2 opposite the bumper beam 1 forattaching the bumper beam assembly to the body of the vehicle, forexample to the longitudinal rails of the vehicle. The bumper beam 1 isdimensioned for extending along a major part of the width of the vehiclein the transversal direction. According to an embodiment, the bumperbeam 1 extends over a distance slightly larger than the distanceseparating the two longitudinal rails of the vehicle, for example over adistance of 70% of the width of the vehicle.

The bumper beam 1 is arcuate in the transversal direction, meaning thatthe bumper beam 1 has a curved shape arranged such that a central part 8extends further towards the exterior of the vehicle than the transversalends 6 of the bumper beam 1. This means that the extrados of the bumperbeam is intended to extend towards the exterior of the vehicle while theintrados is intended to extend towards the interior of the vehicle. Theradius of curvature of the bumper beam 1 is for example inferior orequal to 4000 mm, for example comprised between 2000 mm and 4000 mm. Thecurvature radius can be constant or not along the transversal direction.

The bumper beam 1 is obtained by roll forming a steel sheet 10 (FIG. 6),meaning that the steel sheet is folded and bent into shape as will bedescribed in greater detail subsequently. More particularly, the steelsheet 10 is for example profiled into shape. The steel sheet 10 is madeof a steel having a tensile strength greater or equal to 980 MPa, forexample greater than 1500 MPa or greater than 1700 MPa. The steel forexample comprises at least 35% of martensite or of bainite. According toan embodiment, the steel is for example a fully martensitic steel havinga tensile strength of 1500 MPa. The steel can be coated, for examplewith a Zinc or Aluminum based coating. Alternatively, the steel is leftuncoated. The steel sheet 10 has a thickness comprised between 0.8 mmand 1.5 mm, for example around 1 mm. The thickness of the steel sheet isnot necessarily constant, in order to form portions of differentthicknesses in the bumper beam 1.

The steel sheet 10 is folded in such a manner that the bumper beam 1comprises an upper beam 12 and a lower beam 14 each extending in thetransversal direction, the upper beam 12 extending above the lower beam14 in the elevation direction of the vehicle.

The upper beam 12 comprises a front wall 16 intended to extend towardsthe exterior of the vehicle, a rear wall 18 substantially parallel tothe front wall 16 and intended to extend towards the interior of thevehicle and an upper wall 20 joining the upper end of the front wall 16to the upper end of the rear wall 18.

The lower beam 14 comprises a front wall 22 intended to extend towardsthe exterior of the vehicle, a rear wall 24 substantially parallel tothe front wall 22 and intended to extend towards the interior of thevehicle and an lower wall 26 joining the lower end of the front wall 22to the lower end of the rear wall 24.

According to a first embodiment shown in FIGS. 1 to 3, the bumper beam 1further comprises a central wall 28 joining the front walls 16, 22 tothe rear walls 18, 24 of the upper and lower beams 12, 14 and extendingbetween the upper wall 20 of the upper beam 12 and the lower wall 26 ofthe lower beam 14. The central wall 28 therefore forms both a lower wallof the upper beam 12 and an upper wall of the lower beam 14 and iscommon to the upper and lower beams 12, 14, as shown in FIGS. 2 and 3.

According to a second embodiment shown in FIGS. 4 and 5, the upper beam12 comprises a lower wall 27 and the lower beam 14 comprises an upperwall 29 separate from the lower wall 27 of the upper beam 12. The lowerwall 27 of the upper beam 12 is separated from the upper wall 29 of thelower beam 14 by a space 31.

The front wall 16 of the upper beam 12 and the front wall 22 of thelower beam 14 extend substantially in a same plane and the rear wall 18of the upper beam 12 and the rear wall 24 of the lower beam extendsubstantially in a same plane parallel to the plane of the front walls16, 22. In the mounted state, the planes of the front walls 16, 22 andof the rear walls 18, 24 are planes containing an elevation and atransversal directions, corresponding substantially to vertical planes.The distance between the front walls 16, 22 and the rear walls 18, 24 isfor example around 30 mm.

The upper wall 20 of the upper beam 12 and the lower wall 26 of thelower beam 14 are for example substantially parallel to each other andfor example substantially perpendicular to the planes of the front walls16, 22 and of the rear walls 18, 24. In the mounted state, the planes ofthe upper wall 20 and of the lower wall 26 are planes containing alongitudinal and a transversal directions, corresponding substantiallyto horizontal planes. The distance between the upper wall 20 of theupper beam 12 and the lower wall 26 of the lower beam 14 is for examplearound 120 mm.

According to the first embodiment, the central wall 28 extendssubstantially at the same distance from the upper wall 20 of the upperbeam 12 and from the lower wall 26 of the lower beam 14 such that theupper beam 12 and the lower beam 14 have substantially the samedimensions and equal cross-sections. According to a variant, the centralwall 28 could extend at different distances from the upper wall 20 andlower wall 26, such that one of the cross-sections of the upper beam 12and of the lower beam 14 is greater than the other cross-sections.

According to the first embodiment, the bumper beam 1 therefore has across-section in a plane perpendicular to the transversal directionhaving an 8-shape as shown in FIGS. 2 and 3. However, the cross-sectionof the bumper could be different, for example by having non parallelfront and rear walls and/or non-parallel upper and lower walls.

According to a particularly advantageous variant of the firstembodiment, the central wall 28 comprises at least one change of planebetween the front walls 16, 22 and the rear walls 18, 24, meaning thatthe central wall 28 extends in at least two different planes. Thecentral wall 28 comprises a front part 30 connected to the front walls16, 22, a rear part 32 connected to the rear walls 18, 24, and a centralpart 34 connecting the front part 30 to the rear part 32. The centralpart 34 extends in a plane different from the plane in which the frontpart 30 and/or the rear part 32 extend. According to the embodimentshown in FIGS. 2 and 3, the front part 30 extends in a first plane, therear part 32 extends in a second plane and the central part 34 extendsin a third plane. The first and second planes are substantially paralleland perpendicular to the planes of the front walls 16, 22 and of therear walls 18, 24. The third plane is inclined between the first planeand the second plane. For example, the third plane forms an angle αcomprised between 10° and 170° with the first and second planes.According to an example, angle α is comprised between 30° and 60°. Thecentral part 34 therefore forms a step in the central wall 28. Duringthe deformation of the bumper beam during an impact, the step in thecentral wall separates the central wall in two parts extending indifferent planes, which therefore delays the buckling of the two parts.Indeed, the effort to induce buckling of a surface becomes lower as thelength of the surface in the longitudinal direction in which the effortis applied on the bumper beam during the impact increases. Compared to acentral wall extending in a single plane, since providing a step in thecentral wall makes it possible to reduce the length of the surface ofthe first and second parts of the central wall in the longitudinaldirection, buckling of the first and second parts is delayed since theeffort to induce buckling is greater. Delaying the buckling allowsmaintaining the hollow bodies of the upper beam 12 and of the lower beam14 over a greater distance of deformation which improves theperformances of the bumper beam during the impact in terms of absorbedenergy.

The central part 34 is connected to the front part 30 and to the rearpart 32 by radiused portions 36, i.e. curved portions making thetransition between the first plane and the third plane and between thethird plane and the second plane. The radius of curvature of theradiused portions 36 is greater or equal to 0.5 times the thickness ofthe steel sheet 10. According to an example, the radius of curvature ofthe radiused portions 36 is greater or equal to two times the thicknessof the steel sheet 10. According to the example mentioned previously,the radius of curvature of the radiused portions 36 is therefore greateror equal to a value between 1.6 mm and 3 mm depending on the thicknessof the steel sheet 10.

The first part 30 extends for example at a height different than theheight of the second part 32, meaning that the distance between thefirst part 30 and the upper wall 20 of the upper beam 12, respectivelythe lower wall 26 of the lower beam 14, is different than the distancebetween the second part 32 and the upper wall 20 of the upper beam 12,respectively the lower wall 26 of the lower beam 14. According to theembodiment shown in FIGS. 2 and 3, the first distance between the firstpart 30 and the upper wall 20 of the upper beam 12 is less than thesecond distance between the second part 32 and the upper wall 20 of theupper beam 12, meaning that the first part 30 extends higher than thesecond part 32 in the mounted state of the bumper beam. According to anembodiment, the difference between the first distance and the seconddistance is less than a third of the distance separating the upper wall20 of the upper beam 12 from the lower wall 26 of the lower beam 14,i.e. less than 40 mm according to the example described previously.According to an embodiment, the difference between the first distanceand the second distance, corresponding to the distance between the firstplane and the second plane, is around 10 mm.

The first part 30 is connected to the front wall 16 of the upper beam 12by a radiused front end 38 and the second part 32 is connected to therear wall 24 of the lower beam 14 by a radiused rear end 40. Like theradiused portions 36 between the central part 36 and the front and rearparts 30, 32, the radius of curvature of the radiused front and rearends 38, 40 is greater or equal to 0.5 times the thickness of the steelsheet 10. According to an example, the radius of curvature of theradiused ends 38, 40 is greater or equal to two times the thickness ofthe steel sheet 10.

According to the embodiment shown in FIGS. 1 and 2, the central wall 28has a center of symmetry at the center of the central wall in thelongitudinal direction.

The central wall 28 extending substantially at the center of the bumperbeam 1 increases the resistance of the bumper beam 1. Consequently, thebumper beam 1 can resist to a greater peak effort during an impact.Furthermore, the step formed by the central part 34 delays buckling ofthe surfaces extending in a single plane during the impact, which makesthe bumper beam 1 able of absorbing a greater quantity of energy duringthe deformation of the bumper beam 1 and less likely to break duringdeformation as explained previously. Consequently, the central wall 28improves the characteristics of the bumper beam.

In the diagram shown in FIG. 6 showing the force applied to the bumperbeam relative to the distance of deformation of the bumper beam, thepeak effort is shown on the force ordinate at “Ep”. The distance ofdeformation at which a minimal energy is still absorbed without thebumper beam breaking is shown on the distance ordinate at Em. This canbe obtained with a central wall 28 according to the above embodiment ofthe invention, because with such a central wall, it is for examplepossible to delay the buckling of the central wall 28 by 10 ms to 20 msduring a front impact against the bumper beam occurring at a speed of 15Km/h.

It should be understood that the shape of the central wall 28 could bedifferent. According to an example, the first and second parts 30, 32 ofthe central wall could extend in a same plane while the central part 34could extend in more than one plane. The front end 38 could be connectedto the front wall 22 of the lower beam 14 while the rear end 40 could beconnected to the rear wall 18 of the upper beam 12. The first part 30could extend at a height inferior to the height of the second part 32.

The steel sheet 10 extends between a first edge 42 and a second edge 44,as shown in FIG. 4. When the bumper beam 1 is formed, the first edge 42is attached to the front wall 16 of the upper beam 12 and covers theradiused front end 38 of the central wall 28 and the second edge 44 isattached to the rear wall 24 of the lower beam 14 and covers theradiused rear end 40 of the central wall 28. Consequently, the first andsecond edges 42 and 44 close the cross-sections of the upper and lowerbeams 12, 14. The first and second edges 42, 44 extend in planesparallel to the planes of the front walls 16, 22 and rear walls 18, 24such that planar surfaces are attached together when the edges areattached to the front and rear walls. This makes the closing of thecross-sections an easier step. For example, the first and second edges42, 44 are attached by welding the front and rear walls 16, 24 andwelding flat surfaces is easier than welding curved surfaces. Thewelding is preferably a laser welding.

According to the second embodiment, the distance separating the upperwall 20 and the lower wall 27 of the upper beam 12 and the distanceseparating the upper wall 29 and the lower wall 26 of the lower beam 14are substantially equal such that the upper beam 12 and the lower beam14 have substantially the same dimensions and equal cross-sections.According to a variant, the distance separating the upper wall 20 andthe lower wall 27 of the upper beam 12 and the distance separating theupper wall 29 and the lower wall 26 of the lower beam 14 are differentsuch that one of the cross-sections of the upper beam 12 and of thelower beam 14 is greater than the other cross-sections.

The space 31 substantially extends in the longitudinal direction fromthe plane of the rear walls 18, 24 to the plane of the front walls 16,22, meaning that the space 31 extends substantially over the whole widthof the bumper beam in the longitudinal direction. In the elevationdirection, the space 31 for example has a height substantially comprisedbetween a third and half of the height of one of the rear walls 18, 24.The space 31 is delimited by a junction wall 46 extending between thelower wall 27 of the upper beam 12 and the upper wall 29 of the lowerbeam 14, the junction wall 46 extending substantially in the plane ofthe front walls 16, 22. In the plane of the rear walls 18, 24, the space31 is opened towards the exterior of the bumper beam 1.

According to the second embodiment, the bumper beam 1 therefore has across-section in a plane perpendicular to the transversal directionhaving a B-shape, as shown in FIGS. 4 and 5. However, the cross-sectionof the bumper could be different, for example by having non parallelfront and rear walls and/or non-parallel upper and lower walls. Thecross-section could also be different by having a bend in the junctionwall 46.

According to a variant shown in FIG. 4, the lower wall 27 of the upperbeam 12 and the upper wall 29 of the lower beam 14 are substantiallyplanar and substantially parallel to each other or slightly divergingfrom each other towards the rear walls 18, 24.

According to a particularly advantageous variant of the secondembodiment shown in FIG. 5, each of the lower wall 27 of the upper beam12 and the upper wall 29 of the lower beam 14 comprises at least onechange of plane between the corresponding front wall 16, 22 and thecorresponding rear wall 18, 24, meaning that each of the lower wall 27of the upper beam 12 and the upper wall 29 of the lower beam 14 extendsin at least two different planes. Each of the lower wall 27 of the upperbeam 12 and the upper wall 29 of the lower beam 14 comprises a frontpart 48 connected to the corresponding front wall 16, 22, a rear part 50connected to the corresponding rear wall 18, 24, and a central part 52connecting the front part 48 to the rear part 50. The central part 52extends in a plane different from the plane in which the front part 48and/or the rear part 50 extends. According to the embodiment shown inFIG. 5, each front part 48 extends in a first plane, each rear part 50extends in a second plane and each central part 52 extends in a thirdplane. The first and second planes are substantially parallel andperpendicular to the planes of the front walls 16, 22 and of the rearwalls 18, 24. The third plane is inclined between the first plane andthe second plane. For example, the third plane forms an angle αcomprised between 10° and 170° with the first and second planes.According to an example, angle α is comprised between 30° and 60°. Thecentral part 52 therefore forms a step in each of the lower wall 27 ofthe upper beam 12 and the upper wall 29 of the lower beam 14. During thedeformation of the bumper beam during an impact, the step in the in eachof the lower wall 27 of the upper beam 12 and the upper wall 29 of thelower beam 14 separates these walls in two parts extending in differentplanes, which therefore delays the buckling of the two parts. Indeed,the effort to induce buckling of a surface becomes lower as the lengthof the surface in the longitudinal direction in which the effort isapplied on the bumper beam during the impact increases. Compared towalls extending in a single plane, since providing a step in the centralwall makes it possible to reduce the length of the surface of the firstand second parts of the lower wall 27 of the upper beam 12 and of theupper wall 29 of the lower beam 14 in the longitudinal direction,buckling of the first and second parts is delayed relative since theeffort to induce buckling is greater. Delaying the buckling allowsmaintaining the hollow bodies of the upper beam 12 and of the lower beam14 over a greater distance of deformation which improves theperformances of the bumper beam during the impact in terms of absorbedenergy. Each central part 52 is connected to the corresponding frontpart 48 and to the corresponding rear part 50 by radiused portions 54,i.e. curved portions making the transition between the first plane andthe third plane and between the third plane and the second plane. Theradius of curvature of the radiused portions 54 is greater or equal to0.5 times the thickness of the steel sheet 10. According to an example,the radius of curvature of the radiused portions 54 is greater or equalto two times the thickness of the steel sheet 10. According to theexample mentioned previously, the radius of curvature of the radiusedportions 54 is therefore greater or equal to a value between 1.6 mm and3 mm depending on the thickness of the steel sheet 10.

The first part 48 extends for example at a height different than theheight of the second part 50. For the lower wall 27 of the upper beam12, this means that the distance between the first part 48 and the upperwall 20 of the upper beam 12 is different from the distance between thesecond part 50 and the upper wall 20 of the upper beam 12. According tothe embodiment shown in FIG. 5, the first distance between the firstpart 48 of the lower wall 27 and the upper wall 20 of the upper beam 12is greater than the distance between the second part 50 of the lowerwall 27 and the upper wall 20 of the upper beam 12. For the upper wall29 of the lower beam 14, the distance between the first part 48 and thelower wall 26 of the lower beam 14 is different from the distancebetween the second part 50 and the lower wall 26 of the lower beam 14.According to the embodiment shown in FIG. 5, the first distance betweenthe first part 48 of the upper wall 29 and the lower wall 26 of thelower beam 14 is greater than the distance between the second part 50 ofthe upper wall 29 and the lower wall 26 of the lower beam 14.Consequently, according to the embodiment shown in FIG. 5, the distancebetween the first parts 48 of the lower wall 27 of the upper beam 12 andof the upper wall 29 of the lower beam 14 is inferior to the distancebetween the second parts 50 of the lower wall 27 of the upper beam 12and of the upper wall 29 of the lower beam 14. This means that the thirdplanes of the central parts 52 of the lower wall 27 of the upper beam 12and of the upper wall 29 of the lower beam 14 are diverging planes.According to an embodiment, the difference between the first distanceand the second distance is less than a third of the distance separatingthe upper wall 20 of the upper beam 12 from the lower wall 26 of thelower beam 14. According to an embodiment, the difference between thefirst distance and the second distance is for example comprised betweenthe thickness of the steel sheet and 15 mm, for example around 6 mm.

The first part 48 of the lower wall 27 of the upper beam 12 is connectedto the front wall 16 of the upper beam 12 by a radiused front end 56 andthe second part 50 is connected to the rear wall 18 of the upper beam 12by a radiused rear end 58. The first part 48 of the upper wall 29 of thelower beam 14 is connected to the front wall 22 of the lower beam 14 bya radiused front end 60 and the second part 50 is connected to the rearwall 24 of the lower beam 14 by a radiused rear end 62. Like theradiused portions 54 between the central part 52 and the front and rearparts 48, 50, the radius of curvature of the radiused front and rearends 56, 58, 60, 62 is greater or equal to 0.5 times the thickness ofthe steel sheet 10. According to an example, the radius of curvature ofthe radiused ends 56, 58, 60, 62 is greater or equal to two times thethickness of the steel sheet 10.

According to the embodiment shown in FIG. 5, the lower wall 27 of theupper beam 12 and the upper wall 29 of the lower beam 14 each have acenter of symmetry at the center of the wall in the longitudinaldirection.

The lower wall 27 of the upper beam 12 and the upper wall 29 of thelower beam 14 extending substantially at the center of the bumper beam 1increase the resistance of the bumper beam 1, to an effort appliedagainst the bumper beam 1 during an impact. Consequently, the bumperbeam 1 can absorb a greater maximum effort applied during an impact.Furthermore, the step formed by the central parts 52 delays buckling ofthe surfaces extending in a single plane during the impact, which makesthe bumper beam 1 able absorbing a greater quantity of energy during thedeformation of the bumper beam 1 and less likely to break duringdeformation, as explained previously. Consequently, the lower wall 27 ofthe upper beam 12 and the upper wall 29 of the lower beam 14 improve thecharacteristics of the bumper beam.

It should be understood that the shape of the lower wall 27 of the upperbeam 12 and the upper wall 29 of the lower beam 14 could be different.According to an example, the first and second parts 48, 50 of the lowerwall 27 of the upper beam 12 and the upper wall 29 of the lower beam 14could extend in a same plane while the central part 52 could extend inmore than one plane. The distance between the first parts 48 of thelower wall 27 of the upper beam 12 and the upper wall 29 of the lowerbeam 14 could be superior to the distance between the second parts 50 ofthe lower wall 27 of the upper beam 12 and the upper wall 29 of thelower beam 14. The cross-sections of the upper beam 12 and of the lowerbeam 14 could be different from each other.

According to the embodiment shown in FIGS. 4 and 5, the first edge 42and the second edge 44 of the steel sheet 10 are both attached to thejunction wall 46 to close the cross-sections of the upper beam 12 and ofthe lower beam 14. The first edge 42 covers the radiused front end 56between the lower wall 27 of the upper beam 12 and the second edge 44covers the radiused front end 60 between the upper wall 29 of the lowerbeam 14. The first and second edges 42, 44 extend in the same planeparallel to the plane of the junction wall 46 such that planar surfacesare attached together when the edges are attached to the junction wall.This makes the closing of the cross-sections an easier step. Forexample, the first and second edges 42, 44 are attached by welding thejunction wall 46 and welding flat surfaces is easier than welding curvedsurfaces. The welding is preferably a laser welding.

According to the first and second embodiments described above, the frontwalls 16, 22 of the upper and lower beams 12, 14 each comprise a frontrib 64 extending transversally along the whole length of the bumper beam1. Each front rib 64 has a groove or channel shape extending from thefront wall towards the interior of the bumper beam, i.e. inside thecross-section of the bumper beam towards the rear wall extendingopposite the front wall on which the front rib 64 is provided. Asalready known such front ribs 64 increase the impact strength value ofthe upper and lower beams 12, 14 making the bumper beam 1 able tosustain important maximal efforts during the impact. Each front rib 64has an arcuate shape. According to an embodiment, the radius ofcurvature of each front rib 64 is equal or greater than 0.5 times thethickness of the steel sheet 10. According to an example, the radius ofcurvature of each front rib 64 is greater or equal to two times thethickness of the steel sheet 10. Each front rib 64 extends substantiallyat the center of the front wall 16, 22 in the elevation direction.According to an embodiment, the height of the front rib 64, i.e. thedimension of the front rib 64 measured in the elevation direction issubstantially comprised between a 10% and half of the height of thefront wall on which the front rib extends. The height of the front ribis for example comprised between 10 mm and 30 mm. The depth of the frontrib 64, i.e. the dimension of the rib measured in the longitudinaldirection is for example comprised between a tenth and a third of thedistance between the front wall on which the rib extends and the rearwall facing the front wall. For example, the depth of the front rib 64is comprised between 3 mm and 10 mm. According to a particular example,the height of the rib is equal to the depth of the rib. The front rib 64extending on the front wall 16 of the upper beam 12 is for examplesubstantially identical to the front rib 64 extending on the front wall22 of the lower beam 14. According to various embodiments, at least oneof the upper wall 20 of the upper beam 12, the lower wall 26 of thelower beam 14, the rear wall 18 of the upper beam 12 and the rear wall24 of the lower beam 14 further comprises a rib extending transversallyand towards the interior of the bumper beam 1. It is understood by “arib extending transversally and towards the interior of the bumper beam”that the rib extends inside the cross-section of the bumper beam.

According to an embodiment particularly advantageous when the rear walls18, 24 have an important height, the rear walls 18, 24 of the upper andlower beams 12, 14 each comprise a rear rib 66 extending transversallyalong the whole length of the bumper beam 1, as shown in FIGS. 2 and 3.Each rear rib 66 has a groove or channel shape extending from the rearwall towards the interior of the bumper beam, i.e. inside thecross-section of the bumper beam towards the front wall extendingopposite the rear wall on which the rear rib 66 is provided. It shouldbe noted that the rear ribs 66 have been represented on the bumper beam1 according to the first embodiment but that the rear ribs 66 could alsobe provided in the bumper beam 1 according the second embodiment.

The rear ribs 66 are provided for improving the production of the bumperbeam 1 in order to obtain a bumper beam of improved quality. Asdescribed previously, the bumper beam 1 is curved and the intradosextends on the rear walls 18, 24 side of the bumper beam 1. Large planarsurfaces on the intrados side, such as the surfaces formed by the rearwalls 18, 24 without the ribs, tend to buckle during the bending of thebumper beam 1 performed to curve the bumper beam 1 in the transversaldirection. This buckling creates waves in the planar surfaces whichtherefore do not remain planar after bending. This phenomenon is greateras the radius of curvature of the bumper beam is smaller. As explainedpreviously, this buckling is problematic in terms of integration of thebumper beam in its environment and of attachment of the crash-boxes.

Providing ribs 66 on the rear walls 18, 24 allows reducing thedimensions of the planar surfaces forming the rear walls 18, 24 byreducing the height of the planar surfaces measured in the elevationdirection. Consequently, thanks to the rear ribs 66, buckling of therear walls 18, 24 can be avoided during the bending of the bumper beam.Indeed, the effort to induce buckling of a surface becomes lower as thelength of the surface in the elevation direction increases. Sinceproviding a rib in the rear wall makes it possible to reduce the lengthof the planar surfaces in the elevation direction, buckling of theplanar surfaces can be avoided because the effort to induce buckling isgreater and remains inferior to the effort applied on the bumper beamduring the bending of the bumper beam.

To this end, the height and position of each rear rib 66 on a rear wallis arranged such that the planar surfaces 68 extending on either side ofthe rear rib 66 does not have a height sufficient to cause bucklingduring the bending of the bumper beam. According to an example, theheight of each planar surface 68 does not exceed the half of the heightof the rear face on which the rib 66 extends. Each rear rib 66 forexample extends substantially at the center of the rear wall 18, 24 inthe elevation direction. The height of each rear rib 66 is for examplesubstantially comprised between a third and half of the height of thefront wall on which the front rib extends. For rear faces having alarger height, it could be advantageous to provide more than one rearrib on said rear faces to limit the height of each planar surface ofsaid rear faces such that buckling of rear faces can be avoided duringbending of the bumper beam 1. According to an embodiment, the rear ribs66 are arranged such that the height of each planar surface is inferioror equal to 30 mm.

Each rear rib 66 has an arcuate shape. According to an embodiment, theradius of curvature of each rear rib is equal or greater than 0.5 timesthe thickness of the steel sheet 10. According to an example, the radiusof curvature of each rear rib is greater or equal to two times thethickness of the steel sheet 10. The depth of each rear rib can be ofany value greater than or equal to 0.5 times the thickness of the steelsheet 10 depending on the wanted geometry of the rear wall. According toan embodiment, the depth can be such that the rear rib 48 extends up tothe front wall extending opposite the rear wall on which the rear ribextends or up to the front rib 64 when the rear rib 66 extends oppositea front rib 64. According to the embodiment shown in the figures, thedepth of the rear ribs 66 is inferior to the depth of the front ribs 64.The rear ribs 66 can extend opposite the front ribs 64 or can be offsetrelative to the front ribs 64 in the elevation direction.

Moreover, a rear rib 66 extending transversally and towards the interiorof the bumper beams avoids the risk of a perforation of the radiatorwhich could occur with a rib extending transversally and towards theexterior of the bumper beam, for example during a crash pushing thebumper beam against the radiator.

According to an embodiment, which can be an alternative to or can beimplemented with the embodiments described above, the upper wall 20 ofthe upper beam 12 and/or the lower wall 26 of the lower beam 14 comprisea reinforcement rib 70 which extends transversally along the wholelength of the bumper beam 1. It should be noted that the reinforcementribs 70 have been represented on the bumper beam 1 according to thesecond embodiment but that the reinforcement ribs 70 could also beprovided in the bumper beam 1 according the first embodiment.

The reinforcement rib 70 extends transversally and towards the interiorof the bumper beam.

The reinforcement rib 70 has substantially the same effect as the frontribs 64 and improves the performances of the bumper beam 1. Furthermore,the reinforcement rib 70 can also be advantageous to reduce bucklingrisks in the upper wall 20 of the upper beam 12 and/or the lower wall 26of the lower beam 14. The reinforcement rib 70 has an arcuate shape.According to an embodiment, the radius of curvature of the reinforcementrib 70 is equal or greater than 0.5 times the thickness of the steelsheet 10. According to an example, the radius of curvature of thereinforcement rib 70 is greater or equal to two times the thickness ofthe steel sheet 10. The depth of each reinforcement rib 70, i.e. thedimension of the reinforcement rib 70 in the elevation direction, can beof any value greater than or equal to 0.5 times the thickness of thesteel sheet 10 depending on the wanted geometry of the wall on which therib extends. However, the reinforcement rib 70 is preferably arrangednot to interfere with the front and/or rear ribs or with the centralwall 28 or with the lower wall 27 of the upper beam 12 or with the upperwall 29 of the lower beam 14. According to an embodiment, the depth ofthe reinforcement rib 70 is less than a third of the total height of thebumper beam 1. The reinforcement rib for example extends in thelongitudinal direction at the center of the wall on which the ribextends. The reinforcement rib 70 allows improving the performances ofthe bumper beam 1 in terms of resistance to breaking and in terms ofabsorption of a maximal effort by creating a further deformationpattern.

According to the embodiment shown in FIGS. 1 and 3, which could also beimplemented in the bumper beam 1 according to the second embodiment, thebumper beam 1 further comprises a reinforcement element 72 made ofanother roll formed or stamped steel sheet attached to the upper wall 20of the upper beam 12 and to the lower wall 26 of the lower beam 14 andextending in front of the front walls 16, 22. The reinforcement element72 extends in transversal direction over at least a part of the bumperbeam 1 to form an impact surface in front of at least a part of thefront walls 16, 22. The reinforcement element 72 is arranged to form atleast one cavity 74 with the front walls 16, 22, the cavity 74 extendingbetween the front walls 16, 22 and the reinforcement element 72.According to the embodiment shown in the figures, the reinforcementelement 72 comprises an upper wall 76 defining, with the front wall 16of the upper beam 12, an upper cavity 74 a and a lower wall 78 defining,with the front wall 22 of the lower beam 14, a lower cavity 74 b.Between the upper wall 76 and the lower wall 78, the reinforcementelement 72 comprises a central wall 80 applied against the front walls16, 22 opposite the central wall 28 of the bumper beam 1 or opposite thelower wall 27 of the upper beam 12 and the upper wall 29 of the lowerbeam 14. The reinforcement element 72 can comprise ribs 82 extendingtransversally in the upper wall 76 and/or the lower wall 78.

The reinforcement element 72 allows improving the energy absorption ofthe bumper beam 1 by forming a supplementary deformable structure infront of the bumper beam 1. To this end, the reinforcement element 72extends preferably where additional energy has to be absorbed and whereadditional space in the longitudinal direction is available in front ofthe bumper beam 1 since the reinforcement element 72 increases thecross-section of the bumper beam in the longitudinal direction. Forexample, the reinforcement element 72 extends in the transversaldirection around the center of the bumper beam 1 where most of theenergy of an impact is applied in case of a full front impact againstthe vehicle. The product of the cross-section of the cavity 74 with thetensile strength of the steel forming the reinforcement element 72 andthe thickness of the steel sheet forming the reinforcement element isinferior to the product of cross-section of the bumper beam 1 withoutthe reinforcement element with the tensile strength of the steel formingthe bumper beam and the thickness of the steel sheet such that thebumper beam with the reinforcement element 72 locally absorbs moreenergy than the rest of the bumper beam. For example, the reinforcementis made of a steel which is more ductile than the steel of the bumperbeam.

According to an example, the reinforcement element extends along between10% and the two thirds of the length of the bumper beam 1 in thetransversal direction and the cavity 74 has a cross-sectionsubstantially equal to a third of cross-section of the bumper beamwithout the reinforcement element in the longitudinal direction. Thereinforcement element 72 is for example made of a dual phase steelhaving a tensile strength substantially comprised between 780 and 1500MPa and has a thickness for example equal to the thickness of the steelsheet 10. The reinforcement element 72 is for example laser welded tothe bumper beam 1.

The reinforcement element 72 can also be used to adapt the geometry ofthe bumper beam 1 to particular geometry requirements of differentvehicles. For example, the height of the reinforcement element 72 can begreater than the height of the bumper beam 1 such that the bumper beam 1can be used in vehicles having a larger height than standard vehicles.In this case, the reinforcement element 72 can extend along the wholelength of the bumper beam 1. Consequently, the reinforcement element 72can be used to adapt the bumper beam 1 to a large range of vehicleswherein the bumper beam 1 remains the same for all vehicles and only thereinforcement element is modified to be compliant with the requirementsof the vehicle.

The method for forming the bumper beam 1 described above is partiallyrepresented in FIG. 6 where the successive roll forming steps of thesteel sheet 10 to form a bumper beam according to the first embodimentare represented and labelled 0 to 23. These twenty three roll formingsteps correspond to the number of steps required to form a bumper beam 1having a central wall 28 with a step, which is formed in roll formingsteps 4 to 7, as more clearly visible in FIG. 5, and front ribs 64extending in the front walls 16, 22 and rear ribs 68 in the rear walls18, 24 of the upper beam 12 and of the lower beam 14, which are formedat least in roll forming steps 1 to 3. The various roll forming stepsare performed in successive roll forming stations.

At the end of the roll forming steps, the edges 42 and 44 of the steelsheet 10 are welded to the corresponding front and rear walls or to thejunction wall 46 and the bumper beam 1 is arched in the transversaldirection to obtain its arcuate shape. Thanks to the rear ribs 68, thisoperation does not cause the rear walls 18, 24 to buckle even when theradius of curvature of the bumper beam 1 is reduced and the height ofthe rear walls is important.

When the bumper beam 1 comprises a reinforcement element 72, thisreinforcement element 72 is formed separately for example by rollforming or by stamping, and is attached to the formed bumper beam 1.

The bumper beam disclosed above having an 8-shaped cross section, with athird plane forming an angle α substantially equal to 45° with the firstand second planes, is for example able to be deformed in thelongitudinal direction on a distance greater than 200 mm withoutbreaking. The peak effort sustained by the bumper beam is greater than30 KN, for example around 33 KN (Ep in FIG. 6) and the minimal energyabsorbed after a deformation of 250 mm (Em in FIG. 6) is greater than5.5 KJ, for example around 5.75 KJ. Consequently, the bumper beam 1presents good performances in all three relevant parameters, i.eresistance to a peak effort greater than an effort threshold, minimalenergy absorbed and resistance to breaking.

According to an embodiment of the method for forming the bumper beam,which can be implemented with the embodiments described above, themethod for forming the bumper beam comprises roll forming steps (notshown) for forming in the upper wall 20 of the upper beam 12 and/or inthe lower wall 26 of the lower beam 14 the reinforcement rib 70extending transversally and towards the interior of the bumper beam. Theroll forming steps for forming the reinforcement rib 70 in the upperwall 20 of the upper beam 12 and/or in the lower wall 26 of the lowerbeam 14 are performed in successive roll forming stations.

The ribs extending transversally and towards the interior of the bumperbeam make it possible to limit the volume occupied by the bumper beam.Indeed, the depth of the vehicle (i.e. the dimension of the vehiclemeasured in the longitudinal direction) should not exceed 50 mm to 60mm, otherwise the vehicle could be too long.

Moreover, the ribs extending transversally and towards the interior ofthe bumper beam are more satisfactory than ribs extending transversallyand towards the exterior of the vehicle. Indeed, a rib extending towardsthe outside of the vehicle forms a protrusion where the effortsconcentrate in case of an impact. In the case of a rib directed towardsthe interior of the bumper beam, the efforts are divided on the twolarge surfaces extending on either side of the rib. The risk of fractureof the bumper beam is thus reduced.

What is claimed is:
 1. A bumper beam for an automotive vehicle made ofat least one rolled formed steel sheet comprising an upper beam and alower beam extending in a transversal direction, said upper beam andsaid lower beam each having a closed cross-section defined by a frontwall, a rear wall, an upper wall and a lower wall, the upper wall andthe lower wall joining the front wall to the rear wall, wherein each ofthe front walls of the upper beam and of the lower beam comprises afront rib extending transversally and towards the interior of the bumperbeam, wherein at least one of the upper wall of the upper beam, thelower wall of the lower beam, the rear wall of the upper beam and therear wall of the lower beam further comprises a rib extendingtransversally and towards the interior of the bumper beam, wherein aspace extends between the lower wall of the upper beam and the upperwall of the lower beam, and the lower wall of the upper beam and/or theupper wall of the lower beam extends in at least two different planes.2. The bumper beam according to claim 1, wherein at least one of therear walls of the upper beam and of the lower beam comprises a rear ribextending transversally and towards the interior of the bumper beam. 3.The bumper beam according to claim 2, wherein the rear rib has a curvedcross-section having a radius of curvature greater or equal to 0.5 timesthe thickness of the steel sheet.
 4. The bumper beam according to claim2, wherein the rear wall on which the rear rib extends comprises atleast two planar surfaces extending on either side of the rear rib, theposition and/or the height of the rear rib being arranged such that eachplanar surface has a height inferior or equal to half of the height ofthe rear wall on which the rear rib extends.
 5. The bumper beamaccording to claim 2, wherein the depth of the rear rib is superior orequal to 0.5 times the thickness of the steel sheet and inferior than orequal to the distance between the front wall and the rear wall on whichsaid rear rib extends.
 6. The bumper beam according to claim 2, whereinthe rear rib extends opposite a front rib.
 7. The bumper beam accordingto claim 2, wherein each of the rear walls of the upper beam and of thelower beam comprises a rear rib extending transversally and towards theinterior of the bumper beam.
 8. The bumper beam according to claim 1,wherein the upper wall of the upper beam and/or the lower wall of thelower beam comprises a reinforcement rib extending transversally andtowards the interior of the bumper beam.
 9. The bumper beam according toclaim 8, wherein the reinforcement rib has a curved cross-section havinga radius of curvature greater than or equal to 0.5 times the thicknessof the steel sheet.
 10. The bumper beam according to claim 8, whereinthe depth of the reinforcement rib is superior than or equal to 0.5times the thickness of the steel sheet and inferior or equal to a thirdof the distance between the upper wall of the upper beam and the lowerwall of the lower beam.
 11. The bumper beam according to claim 1,wherein the bumper beam is curved in the transversal direction, thecurvature radius of the bumper beam being less than or equal to 4000 mm.12. Bumper beam according to claim 1, wherein the steel sheet is made ofa steel having a tensile strength greater or equal to 980 Mpa.
 13. Thebumper beam according to claim 12, wherein the steel sheet has athickness substantially comprised between 0.8 mm and 1.5 mm.
 14. Thebumper beam according to claim 1, wherein a central wall extendingbetween the front walls and the rear walls of the upper beam and of thelower beam forms both the lower wall of the upper beam and the upperwall of the lower beam.
 15. The bumper beam according to claim 14,wherein the central wall extends in at least two different planes. 16.The bumper beam according to claim 1, further comprising a reinforcementelement made of a steel sheet, said reinforcement element being attachedto the upper beam and to the lower beam such that the reinforcementelement extends opposite at least a part of the front walls of the upperbeam and of the lower beam and defines with said front walls at leastone cavity extending between said front walls and said reinforcementelement.
 17. A method for producing a bumper beam according to claim 1,comprising the steps of: providing a steel sheet, roll forming the steelsheet in successive rolling stations in order to form a bumper beamcomprising an upper beam and a lower beam extending in a transversaldirection, said upper beam and said lower beam each having a closedcross-section defined by a front wall, a rear wall, an upper wall and alower wall, the upper wall and the lower wall joining the front wall tothe rear wall, wherein each of the front walls of the upper beam and ofthe lower beam comprises a front rib extending transversally and towardsthe interior of the bumper beam, wherein at least one of the upper wallof the upper beam, the lower wall of the lower beam, the rear wall ofthe upper beam and the rear wall of the lower beam is shaped in at leastone of the rolling stations to comprise a rib extending transversallyand towards the interior of the bumper beam, wherein a space extendsbetween the lower wall of the upper beam and the upper wall of the lowerbeam, and wherein the lower wall of the upper beam and/or the upper wallof the lower beam extends in at least two different planes.
 18. Themethod according to claim 17, wherein the steel sheet is made of a steelhaving a tensile strength greater or equal to 980 MPa.